Many medical procedures involve introducing an object into a patient's body and sensing the object's movement. To support these procedures, various position sensing systems have been developed or envisioned in the prior art.
For example, U.S. Pat. Nos. 5,697,377 and 5,983,126 to Wittkampf, whose disclosures are incorporated herein by reference, describe a system in which three substantially orthogonal alternating signals are applied through the patient. A catheter is equipped with a measuring electrode, which for cardiac procedures is positioned at various locations either against the patient's heart wall or within a coronary vein or artery. A voltage is sensed between the catheter tip and a reference electrode, preferably a surface electrode on the patient. Three processing channels are used to separate out the three components as x, y and z signals, from which calculations are made for determination of the three-dimensional location of the catheter tip within the body.
U.S. Pat. No. 5,944,022 to Nardella, whose disclosure is incorporated herein by reference, describes a similar system for detecting the position of a catheter. The system includes three sets of excitation electrodes, with one set disposed in each of three intersecting axes. A signal processor measures a voltage indicative of impedance between a detection electrode disposed on the catheter and each of the three sets of excitation signals in order to determine the X coordinate, Y coordinate and Z coordinate of the catheter.
Additional methods for detecting impedance along axes between excitation electrodes are disclosed by U.S. Pat. No. 5,899,860 to Pfeiffer; U.S. Pat. No. 6,095,150 to Panescu; U.S. Pat. No. 6,456,864 to Swanson; and U.S. Pat. No. 6,050,267 to Nardella, all of whose disclosures are incorporated herein by reference.
Impedance measurements are also used in assessing contact between an electrode and tissue inside the body. For example, methods for determining contact between a catheter electrode and internal tissue, based on the impedance between the catheter electrode and a return electrode, are described in U.S. Pat. No. 5,935,079 to Swanson, et al., U.S. Pat. No. 5,836,990 to Li, U.S. Pat. No. 5,447,529 to Marchlinski, et al., and U.S. Pat. No. 6,569,160 to Goldin, et al., all of whose disclosures are incorporated herein by reference. U.S. Pat. No. 5,341,807 to Nardella, whose disclosure is incorporated herein by reference, describes a system for detecting when an ablation electrode contacts endocardium tissue, utilizing separate circuits for position monitoring and for tissue contact monitoring. When the ablation electrode touches internal tissue, the impedance from the body-surface to the electrode increases because less of the electrode is in contact with the electrolytic fluid (i.e., blood) that generally surrounds the probe.